Real reason network IP address cannot be assigned

Hello,

Only an academic question.

What is the real reason by which the IP network address (e.g. 145.31.0.0)
cannot be assigned to hosts as their IP address? I understand, it is network
address by its definition. But I presume it would work with routers to e.g.
ping some computer with this address assigned (when it finds itself on the
right subnet etc.).

I only found out some idea about problems with ARP, but don't sure about it.

Thanks in advance for your hints.

Nice day
O.